Data from: When anthropogenic-related disturbances overwhelm demographic persistence mechanisms

1. Population decline is associated with increased vulnerability to extinction, but also with possible density-, frequency-, or distance-related ‘rarity advantages’ that increase recruitment success as individuals become isolated from their congeners. Distinguishing between these alternatives (risk versus recovery of rare populations via demographic processes) has become critical, given how anthropogenic disturbances are causing population declines globally. 2. Here, we demonstrate how distance-related rarity advantages are evident in spatially isolated recruits of a canopy-dominant but regionally rare species of oak that appears to be suffering recruitment collapse. As distance from parent trees increased, seedlings had significantly more leaves and experienced reduced insect browsing and intraspecific competition. Long-term field-based experimental treatments revealed these advantages to be associated with rapid rates of juvenile maturation and survival that are unobserved in natural settings. 3. The discrepancy between the experimental and natural settings was explained by trophic collapse and habitat loss - two changes ubiquitous to many terrestrial ecosystems – that combine to concentrate vertebrate herbivores in habitat remnants and cause 100% juvenile mortality via the browsing of taller juveniles. Exotic grass cover, long associated with oak recruitment failure, significantly suppressed seedling height and leaf production, but appeared to delay mortality by hiding shorter seedlings from vertebrate herbivores. 4. Synthesis. Our work demonstrates how rarity advantages have the potential to positively influence the population performance of a declining species, but are short-circuited by intense herbivory associated with human-based environmental change. Regionally, there appear to be few existing conditions on the contemporary landscape that favor juvenile survival, suggesting ongoing recruitment difficulties without intervention. Our work clarifies how extinction risk can in some cases be best defined by how anthropogenic disturbances affect, and are offset by, demographic-based persistence mechanisms, than simply by present-day abundance or distribution.

1. 种群下降 (population decline) 与灭绝 (extinction) 风险升高息息相关，同时还可能伴随密度、频率或距离相关的稀有优势 (rarity advantages)：当个体与同属物种 (congeners) 愈发疏离时，这类优势可提升种群补充成功 (recruitment success) 率。鉴于全球范围内人为干扰 (anthropogenic disturbances) 正持续引发种群下降，区分这两种对立情形——即稀有种群通过种群动态过程面临灭绝风险，或是实现种群恢复——已成为至关重要的研究议题。
2. 本研究证实，在一种冠层优势但区域稀有的栎属 (oak) 物种的空间隔离幼苗 (seedlings) 中，距离相关的稀有优势表现显著；该物种正经历种群补充崩溃 (recruitment collapse)。随着与母树 (parent trees) 距离的增加，幼苗的叶片数量显著增多，且昆虫取食 (insect browsing) 与种内竞争 (intraspecific competition) 压力均有所降低。长期野外实验处理 (field-based experimental treatments) 结果显示，这类优势与幼体快速成熟及高存活率相关，而该现象在自然环境中从未被观测到。
3. 实验环境与自然环境间的差异可通过营养级崩溃 (trophic collapse) 与栖息地丧失 (habitat loss) 得到解释：这两种变化在众多陆地生态系统 (terrestrial ecosystems) 中普遍存在，二者共同促使脊椎动物食草动物 (vertebrate herbivores) 聚集于栖息地残存片段 (habitat remnants)，并通过取食较高的幼体造成100%的幼体死亡率。长期以来与栎属物种补充失败相关的外来草本覆盖物 (exotic grass cover)，显著抑制了幼苗的株高与叶片生成，但似乎通过将矮小幼苗隐藏起来，使其免受脊椎动物食草动物的取食，从而延迟了幼苗死亡。
4. 综合讨论 (Synthesis)。本研究表明，稀有优势可对衰退物种种群表现产生积极影响，但会被人类活动引发的环境变化所伴随的高强度食草作用抵消。从区域尺度来看，当前景观中几乎不存在有利于幼体存活的条件，这意味着若不采取干预措施，种群补充困难的状况将持续存在。本研究阐明，在某些情况下，灭绝风险的最佳定义不应仅依据当下的种群丰度或分布范围，而应考虑人为干扰如何影响基于种群动态的存续机制 (demographic-based persistence mechanisms)，以及这些机制如何抵消人为干扰的影响。